This invention relates to a fuel-firing burner which gives off combustion exhaust gases with reduced nitrogen oxide contents.
Nitrogen oxides (hereinafter called "NOx") in a combustion flame are mostly formed as the oxygen and nitrogen molecules in the air combine as a result of the combustion. The production of NOx depends largely on the flame temperature, the higher the temperature the faster the tempo in which NOx are formed. It is well-recognized, therefore, that lowering the flame temperature is effective in decreasing the NOx production that accompanies the process of combustion.
With ordinary fuel-firing burners, attempts have been made to reduce the NOx contents of their combustion exhaust gases, for example, by
(a) a stepwise combustion method, so called because part of the combustion air is supplied from the burner itself to lower the excess air ratio in the burner and the rest of the excess air is issued from ports independent of the burner, or PA1 (b) an inert gas addition method whereby an inert gas (e.g., the combustion exhaust gas) is mixed in the combustion air. PA1 (1) The burner comprises two or more air nozzles. PA1 (2) The air nozzles are arranged unsymmetrically with respect of the central axis of the burner at its outlet. PA1 (3) The velocity of combustion air to issue from the air nozzles is a combination of velocities from different nozzle groups which velocities are respectively higher and lower than the mean velocity through the overall outlet opening area of the burner. PA1 (4) The lower-velocity air nozzle group is designed to have an outlet opening area accounting for from 30 to 60% of the total outlet opening area of the burner. PA1 (5) At least one fuel nozzle is located within or adjacent the lower-velocity air nozzle. PA1 (6) The amount of air that issues from the lower-velocity air nozzle group is not more than 70% of the theoretical amount of air required for the combustion of the fuel injected. PA1 (7) The combined air-fuel ratio for the two groups is such that the excess fuel ratio is not less than 1.
Of those prior art methods, the former that requires additional ports outside of the burner has disadvantages of high initial investment in equipment and inconvenience in controlling the air supply. For the latter, extra ducting and installation of a fan and other mixing facilities are inevitable.
In an effort to mitigate those drawbacks, a burner as typically illustrated in FIG. 1 has been devised to supply air for stepwise combustion from around the outlet of the burner so that the aforesaid beneficial effect is attained at relatively low equipment cost.
In the arrangement of FIG. 1, most of the combustion air issues from the burner nozzle 1 and mixes and burns with most of the fuel. The balance of the air for stepwise combustion is admitted from ports 2 to mix and burn with the balance of the fuel in the afterstream portion of the flame. If the ports 2 are located close to the burner nozzle 1, the air for stepwise combustion will join the flame rather prematurely and abate the NOx-controlling effect. Therefore, the ports must be kept away from the burner nozzle 1, thus increasing the overall burner dimensions.